1. Field of the Invention
The present invention relates to zinc-comprising-plated high tension steel sheets having superior plating appearance, and more particularly, relates to a hot-dip zinc-comprising-plated high tension steel sheet and a hot-dip zinc-alloy-plated high tension steel sheet having superior plating appearance, and to a high tension steel sheet plated with an zinc-comprising-electroplating layer which has a superior adhesion strength, wherein these high tension steel sheets can be preferably used in the fields of automobiles, construction materials, home appliances, and the like.
2. Description of the Related Art
In recent years, the consumption of high tension steel sheets has been increasing in the fields of automobiles, construction materials, home appliances, and the like. In particular, in the field of automobiles, in order to improve fuel consumption, safety during collisions, and the like, high tension steel sheets have been increasingly demanded. As the high tension steel sheets, steel sheets having compositions containing the elements, such as silicon (Si), manganese (Mn), titanium (Ti), aluminum (Al), and phosphorus (P), may be mentioned (for example, disclosed in Japanese Unexamined Patent Application Publication Nos. 61-291924, 60-17052, Japanese Examined Patent Application Publication Nos. 61-11294, and 63-4899). However, it has been well known that when the content of Si is increased, all compositions mentioned above have Si oxide films formed thereon, and as a result, phosphating treatment cannot be preferably performed and the appearance of the hot-dip zinc plating is degraded. The degradation in the appearance of the hot-dip zinc plating means the generation of so-called bare spots, that is, when hot-dip zinc-plating is performed, some spots on the substrate are not plated with molten zinc. In particular, this phenomenon has been a serious problem in high tension steel sheets containing Si.
In hot-dip zinc-plated high tension steel sheets, the relationship between the plating appearance and the amount of Si oxide film existing between the plating layer and the high tension steel sheet has not been quantitatively studied. As a result, hot-dip zinc-plated high tension steel sheets having superior plating appearance have not been reliably manufactured. Furthermore, particularly in recent years, since light weight automobile bodies and the safety during collisions have been increasingly required, steel sheets containing further increased amounts of Si have been developed, and hence, it becomes more difficult to ensure superior plating appearance when hot-dip zinc-plating of the steel sheets mentioned above is performed.
As described above, concerning a hot-dip zinc-plated high tension steel sheet, the level of the content of the Si oxide film formed between the plating layer and the high tension steel sheet at which a hot-dip zinc-plated high tension steel sheet having superior plating appearance can be obtained has not been known at all. However, a hot-dip zinc-plated high tension steel sheet having further improved plating appearance formed by controlling the amount of this Si oxide film in a predetermined range has been strongly desired.
As described above, it has been already understood that phosphating treatment cannot be preferably performed and the plating appearance of hot-dip zinc-plating is degraded due to the presence of this Si oxide film, and in addition, it has also been known that when zinc-electroplating is performed on a high tension steel sheet, the adhesion strength is decreased due to the presence of this Si oxide film.
Accordingly, in order to remove Si oxide films formed on surfaces, there have been proposed, for example, (1) a method of pickling a steel sheet while it is being brushed in a pickling step before plating (disclosed in Japanese Unexamined Patent Application Publication No. 61-159590), (2) a method of polishing surfaces of steel sheets, subsequently performing a pickling treatment for 10 seconds or less, and then performing electroplating (disclosed in Japanese Unexamined Patent Application Publication Nos. 5-230689 and 5-320981), and (3) a method of pickling steel sheets for 3 to 15 seconds using sulfuric acid at a concentration of 20 wt % or more, hydrochloric acid at a concentration of 13 wt % or more, or hydrofluoric acid at a concentration of 3 wt % or more (disclosed in Japanese Unexamined Patent Application Publication No. 7-126888).
However, the relationship between the adhesion strength of the plating layer and the amount of the Si oxide film existing at the interface between a zinc-electroplating layer and the high tension steel sheet has not been quantitatively studied. As a result, even when the above methods (1) to (3) are used, a high tension steel sheet provided with a zinc-electroplating layer which has a superior adhesion strength has not been reliably manufactured, and hence, troubles have frequently occurred in actual pressing steps in automobile manufacturers. In addition, particularly in recent years, since light weight automobile bodies and the safety during collisions have been increasingly required, steel sheets containing further increased amounts of Si have been developed. However, even though the above methods (1) to (3) have been used, the adhesion strength obtained by performing zinc electroplating on the steel sheets mentioned above has been frequently rejected.
As described above, concerning a zinc-electroplated high tension steel sheet, the level of the content of the Si oxide film formed between the plating layer and the high tension steel sheet at which a zinc-electroplated high tension steel sheet having superior adhesion strength can be obtained has not been known at all. However, a zinc-electroplated high tension steel sheet having further improved adhesion strength formed by controlling the amount of this Si oxide film in a predetermined range has been strongly desired.
Accordingly, an object of the present invention is to provide a zinc-comprising-plated high tension steel sheet having superior plating appearance and a high tension steel sheet provided with a zinc-comprising-electroplating layer having a superior adhesion strength by appropriately controlling the content of a Si oxide film at the interface between the plating layer and the high tension steel sheet.
Through detailed research made by the inventors of the present invention on the plating appearances of a hot-dip zinc-comprising-plated high tension steel sheet and a hot-dip zinc-alloy-plated high tension steel sheet, and on the adhesion strength of a zinc-comprising-electroplating layer provided on a high tension steel sheet, it was understood that although Si contained in the steel had an adverse influence on the plating appearances of a hot-dip zinc-comprising-plated high tension steel sheet and a hot-dip zinc-alloy-plated high tension steel sheet, and on the adhesion strength of a zinc-comprising-electroplating layer provided on a high tension steel sheet, the plating appearance and the adhesion strength were not always determined by the content of Si in the steel sheet. It was also understood that, in addition to the Si content, there were various factors which, in combination, influence the plating appearance and the adhesion strength, for example, annealing conditions or pickling conditions performed before annealing had an influence on the plating appearance, and the annealing conditions, the pickling conditions performed before annealing, the presence of a brushing step or a surface polishing step, pre-dipping conditions in a plating solution, or the like had an influence on the adhesion strength.
Accordingly, in order to fully understand the influences of the plurality of factors mentioned above, through research by the inventors of the present invention using various surface analytical methods, it was discovered that the Si contained in a high tension steel sheet was concentrated on the surface thereof in an annealing step before the high tension steel sheet was dipped in a hot-dip zinc-comprising-plating bath; the amount of Si concentrated on the surface of the steel sheet can be decreased by pickling conditions performed before annealing; and the plating appearance was determined by the amount of the concentrated Si which finally remained on the surface. In addition, concerning the zinc-comprising-electroplating, it was also discovered that the amount of Si concentrated on the surface of the steel sheet was decreased by pickling, brushing, surface polishing, predipping in a plating solution before plating, and the like, and the adhesion strength was determined according to the amount of the concentrated Si which finally remained on the surface.
In addition, through intensive research by the inventors of the present invention in order to measure the amount of concentrated Si which remains on the surface of a steel sheet, it was understood that even if sputtering analysis was performed in the depth direction of a high tension steel sheet provided with a hot-dip zinc-comprising-plating layer or a hot-dip zinc-alloy-plating layer, the peaks of the surface-concentrated Si on the steel surface were vague, and that it was difficult to quantitatively measure the amount of the surface-concentrated Si. Accordingly, the inventors of the present invention found that when sputtering analysis in the depth direction was performed after the hot-dip zinc-comprising-plating layer or the hot-dip zinc-alloy-plating layer was removed by dissolution, an accurate amount of surface-concentrated Si could be quantitatively measured. In addition, it was also found that when the amount of surface-concentrated Si is controlled in a predetermined range, a hot-dip zinc-comprising-plated high tension steel sheet or a hot-dip zinc-alloy-plated high tension steel sheet having significantly superior plating appearance could be obtained, whereby the present invention was made.
Similarly to the above, it was also understood that even if sputtering analysis was performed in the depth direction of a high tension steel sheet provided with a zinc-comprising-electroplating layer thereon, the peaks of the surface-concentrated Si on the steel surface were vague, and that it was difficult to quantitatively measure the amount of the surface-concentrated Si. Furthermore, it was found that when sputtering analysis in the depth direction-was performed after the zinc-comprising-electroplating layer was removed by dissolution, an accurate amount of the surface-concentrated Si could be quantitatively measured. It was also found that when the amount of the surface-concentrated Si is controlled in a predetermined range, a high tension steel sheet provided with a zinc-comprising-electroplating layer having a significantly superior adhesion strength could be obtained, whereby the present invention was made.
In accordance with the understanding described above, the present invention provides a high tension steel sheet comprises 0.1 wt % or more of Si and a zinc-comprising-plating layer provided on the high tension steel sheet, wherein the high tension steel sheet has a surface-concentrated Si index X, defined by the formula below, of 12 or less when a sputtering analysis is performed in the depth direction from the surface of the high tension steel sheet after the zinc-comprising-plating layer is removed by dissolution:
X=(the maximum intensity A of Si on the surface of the high tension steel sheet/the average intensity B of Si in the steel sheet)xc3x97the content of Si in the steel sheet on a wt % basis.
In the high tension steel sheet according to the present invention, the zinc-comprising-plating layer provided on the high tension steel sheet may be a zinc-comprising-electroplating layer. The zinc-comprising-electroplating layer provided on the high tension steel sheet has a superior adhesion strength.
In addition, in the high tension steel sheet according to the present invention, it is preferable that the zinc-comprising-plating layer provided thereon be a hot-dip zinc-plating layer, and the high tension steel sheet have a surface-concentrated Si index X of 10 or less according to the formula described above. Consequently, a hot-dip zinc-plated high tension steel sheet having superior plating appearance can be obtained.
Furthermore, in the high tension steel sheet according to the present invention, it is preferable that the zinc-comprising-plating layer provided thereon be a hot-dip zinc-alloy-plating layer, and the high tension steel sheet have a surface-concentrated Si index X of 6 or less according to the formula described above. Consequently, a hot-dip zinc-alloy-plated high tension steel sheet having superior plating appearance can be obtained.